This study assessed the level of oxidative stress and heat shock proteins (Hsp) induced in gills and liver as well as the histopathological effects in the gills of African catfish, Clarias gariepinus, exposed to runoff water from the Third Mainland Bridge, the busiest bridge in the City of Lagos, Nigeria. The runoff was sampled off the bridge drainage, and the assessment of its physicochemical characteristics in both dry and rainy season indicated a high level of chemical and biological oxygen demand, oil and grease as well as manganese. After a preliminary acute toxicity evaluation, the juveniles of C. gariepinus were exposed to 5, 10, 15, 25, 50 and 100% runoff samples for 30 days. The level of the lipid peroxidation product, malondialdehyde, was significantly different from the control and increased serially with higher concentrations of effluent exposure. There was also an inhibition in the activities of enzymes, superoxide dismutase and catalase as well as significantly lower levels of reduced glutathione after 30 days. Heat shock proteins induced in the catfishes over this period included proteases, sHsps, Hsp 40s, Hsp 60s (for all the fishes-control and exposed) and Hsp 70s (in the exposed fishes only). Histopathological assessments of the gills indicated that the major effects of the runoff exposures were mild to severe thickening and fusion of the gill lamellae, stunting of lamellae, erosion of outer epithelium, epithelial lifting, hyperplasia, oedema and aneurism. The implications of the findings in the Lagos lagoon fishes were discussed.